Integrated circuits are often manufactured on a semiconductor substrate, such as a silicon wafer. The silicon wafer is typically a thin circular plate of silicon that is 150 or 200 or 300 millimeters in diameter and approximately 2 millimeters thick. Typically, a single wafer will have numerous devices which are integrated circuits formed in a lattice pattern. Each device consists of numerous layers of circuitry and a collection of external bonding (and optional testing) pads. The bonding pads are small sites, typically 3 mils square, made usually with aluminum that eventually serve as the device's connections to the pin leads.
Probing involves contacting the bonding pads with probe tips to make an electrical (resistive) connection between the probe tips and the bonding pads or other pads. Accurate positioning of the pads under the tips is necessary both to avoid causing damage to the pads and to maintain the desired contact pressure that ensures good electrical contact (the pins must in fact “scrub” through an outer oxide layer forming over the pads under normal atmospheric conditions).
External disturbances, such floor vibrations (typically at 10-30 Hz), accidental hits to the prober unit, or other sources of induced vibrations can disrupt accurate probe tip to pad positioning. Given the inherent flexural, or non-rigid, properties of the joints and connections between components in the prober system, vibrations may arise in different parts of the unit at different frequencies and amplitudes. Flexural deflections at the connections will cause non-compensatory relative displacements between the wafer surface and the probe tips which usually resemble pins.
Relative displacements can be reduced by increasing the rigidity of the connections, or by providing isolation between the floor and the unit. These methods have the disadvantage that they increase the weight of the system thereby increasing both the cost (by, for example, requiring to change the isolation properties at every installation) and the sensitivity in the frequency domain to such disturbances. Prior art systems include so-called isolation tables, typically very heavy and rigidly bolted to a deep underground foundation. These tables result in reaction cancellation or reactive isolation (for example, as is done for wafer steppers). Active isolation tables also exist that include vibration compensation with dedicated actuators. However, all these prior art systems cost very high sums ($800,000 to a million dollars) and incur complex installation procedures precluding portability of the probing unit (portability can be a great advantage for non-in-house production, for example in foundries). On the other hand, existing portable probing systems, due to their lighter weight, do not incorporate isolation tables and are inherently less rigid. Currently, there are no prior art portable probing systems that incorporate active vibration suppression or compensation. Portable probing units incorporating active disturbance compensation would be highly desirable because the corrections would improve throughput by allowing the system to step from die to die on the wafer much quicker.